Xanth is not on my hit-list, but that's probably because I've never had the poor fortune to read a Peirs Anthony novel.
The Mythadventures series, of which I've only read "Another Fine Myth," would go into my hit-list, albeit probably in the lower half. The plot is mediocre, but what really bugs me are the characters. Everything about them that is supposed to be quirky and humorous, I find nerve-gratingly irritating. Because the comedic aspect is the primary redeeming trait to our cast of morally gray main characters, I have no emotional investment, and just wish they'd get sat on by a giant frog or eaten by Ann Coulter or something. This goes double for Green Fanservice chick. I also find Istvan a pretty pathetic villain, as all of his menace comes from informed ability and off-screen deeds of evil.
The upper echelons of my list are reserved for works I find philosophically offensive AND badly written.
The Muchly All-Women-Want-To-Be-Slaves-and-it-is-a-Crime-Against-Nature-to-Suggest-Otherwise Gunless Meathead Philibuster of Gor, by John Norman, gets an honorary mention. I have fortunately never read these works. The only contact with them has been reviews, wiki articles, the fanservice-filled Outlaw of Gor film on MST3K (which apparently toned down the source materials sexist overtones) and the adroit parody-fic "Houseplants of Gor." That said, I must include it because of the sheer zenith this work has achieved in the realm of popular awfulness. Ignore the inept prose and fondness for "muchly." The author goes beyond the realm of traditional sword-and-sorcery fanservice, author interest, and bondage setups, and puts forth the ideology that all men really want to own property, all women really want to be owned and treated like crap, and the world would be perfect if everyone lived by this. It's one thing to say "this is my kink, and I'm writing about it." It's another to say "this is the law of the universe, and everybody who says their not into it is in denial, and you're ruining our utopia by wanting consensual sex."
The worst kind of badness comes not from porny fanservice, nor from long-winded preachyness, but the union of the two.